add 'gres' option to SlurmSpawner

[Hoeze]

As the name tells, it allows to specify e.g. GPU requests

[rkdarst]

We could debate if we should not add a specific option for everything, since the generic req_options works, too. What do others think?

But since this is here, --gres seems to be rather important anyway (if people go more towards GPUs) so why not. I will add an entry to the changelog and add it to my integration branch.

[rkdarst]

I added a test, I think this is good to go.

[mbmilligan]

We may want to revisit the larger policy around proliferating options and requests to change defaults, but for now this seems both sufficiently generic and well-motivated to accept.